Choosing a timber roof structure and comparing installed costs

A roof frame consists of timber structural members and their joints. Rafters define the slopes, while other members support them and tie the frame together. The structure has to carry its own weight, snow, wind loads and the equipment specified for the roof. Its role extends well beyond supporting the outer covering.

Building to the drawings includes installing the specified joints and bracing. Omitting them can change how the frame behaves. The number of beams alone tells you little about quality. Even additional timber cannot compensate for a break in the load path to the supports.

In a rafter roof, the main load-bearing members are the rafters and their connections. Adding collar beams changes how the rafters act together. In a purlin roof, purlins provide additional rafter support. Coordinate posts and bearings with room layouts and the capacity of the structure below.

Prefabricated trusses can form a different system engineered for the individual building. They cannot be freely altered like a set of loose rafters. When comparing systems, consider the design, intended use of the loft, delivery arrangements and erection work alongside the price per cubic metre of timber.

A habitable loft needs space for rooms, stairs, services and insulation. Posts or diagonal braces can limit how rooms are laid out. Check their positions before buying plans and ordering materials. Removing a support later may require extensive structural alterations.

An unused loft may have a different layout, often with insulation at ceiling level. Do not assume it will be suitable for later conversion to living space. Check the capacity of the loft floor, available dimensions, roof frame and planning requirements. Agree any allowance for a future conversion with the designer.

Tile-profile steel sheets and trapezoidal steel sheets weigh less than many heavier coverings. That can be advantageous when designing the structure. However, reducing member sizes in an existing design requires a calculation. The roof is still subject to snow and wind.

Changing the covering can alter batten spacing, the support details and the roof build-up. Above-rafter PIR insulation needs suitable fasteners and corresponding eaves details. Include photovoltaic panels and plant at the design stage if the roof will carry them. It can be harder to accommodate them once the frame is built.

Specify sections, lengths, timber strength class and how the timber is to be supplied. Are the members just cut to length or fully prepared for erection? Machining, drying and joint preparation affect what is included. The timber markings should allow the delivery to be checked against the design.

Check the actual dimensions after machining and plan storage. Protect the timber from standing water and allow it to ventilate. Avoid ordering far in advance unless a suitable storage area is ready. Storage damage can wipe out any saving on the purchase price.

Compare timber, connectors, anchorage, protective measures and erection. Check transport, unloading and any crane hire, and establish whether battens and counterbattens are included. Two items both labelled ‘roof frame’ may cover quite different work.

Changes of roof plane, dormers and complex joints affect labour costs. A simple gable roof generally needs fewer complicated operations than a more intricate roof of the same area. Renovation adds dismantling and adaptation to existing walls. Check how long the quotation is valid and how changes will be priced.

Inspect before subsequent roof layers conceal the frame. Check geometry, timber, bearings and connections against the drawings. Record any deviations and how they will be corrected. Photos of anchors and joints help later but cannot replace an inspection by a suitably qualified person.

Ask for the material quantities from the schedule and, separately, the full cost of a frame ready for the next trade. The second figure shows whether a lower timber price really reduces the installed cost.

Choose the roof structure together with the shape of the house and the intended loft use. A straightforward structural arrangement and a well-organised delivery offer opportunities to save. Member sizes, supports and connections must follow the calculations.